1. Field of the Invention
The present invention relates to an induction heating apparatus effectively used as a fixing apparatus of an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
In an electromagnetic induction heating apparatus, a magnetic field is applied to a conductive member (electromagnetic induction heating material, induction magnetic material, magnetic-field absorbing material) which is fixed or is movable, and a material to be heated is heated by heat generated by eddy currents induced in the conductive member. For example, the heating apparatus is effective as image heating-fixing apparatuses in electrophotographic, electrostatic recording, and magnetic recording image forming apparatuses in which a recording material (material to be heated) having an unfixed toner image thereon is heated, and the unfixed toner image is thereby heated and fixedaas a permanently fixed image.
In a general type of heating apparatus, when materials to be heated (small materials), which have a width smaller than that of materials (large materials) of the maximum possible width to be passed through the apparatus, are successively passed therethrough and are subjected to heating, since heat is not consumed to heat the materials in a paper nonpassing region of a material heating section of the apparatus, that is, in a portion of a paper feeding region for large materials except for a paper feeding region for small materials, the temperature in the portion rises above the temperature in the paper feeding region, and a so-called xe2x80x9cpaper nonpassing region overheating phenomenonxe2x80x9d occurs, that is, the temperature excessively rises above the allowable temperature.
As a technique of preventing such an overheating phenomenon, division and selective control is exerted on the magnetic-field generating means in the above-described electromagnetic induction heating apparatus, as disclosed in, for example, Japanese Patent Application Laid-Open No. 8-16006.
In this control, a magnetic-field generating means having a length corresponding to the maximum paper feeding width of a material to be heated is composed of several units divided in a direction crossing (intersecting) the paper passing direction, that is, the feeding direction of the material, and the units are selectively controlled to generate a magnetic field according to the width of a material to be heated. In a case in which a large material having a width corresponding to the maximum paper feeding width is used in the apparatus, all the units of the magnetic-field generating means are caused to generate heat so that a region of a conductive member corresponding to the maximum paper feeding width generates heat in accordance with the large material. In a case in which a small material having a width smaller than the maximum paper feeding width is used, some units of the magnetic-field generating means corresponding to a passing region for the small material are caused to generate a magnetic field, and the other parts corresponding to a paper nonpassing region are controlled so as not to generate a magnetic field. Consequently, only a region of the conductive member corresponding to the paper passing region for the small material generates heat, thereby heating the small material without causing the paper nonpassing region overheating phenomenon.
FIG. 8 shows an example of the control method. Referring to FIG. 8, the first to third divided exciting coil units A, B, and C serving as divided magnetic-field generating means include coils (exciting coils) 20a, 20b, and 20c and magnetic cores (exciting iron cores) 21a, 21b, and 21c, respectively. A fixed or movable conductive member performs electromagnetic induction heating due to the actions of magnetic fields generated by the first to third divided exciting coil units A, B, and C, and a material to be heated is conveyed into a material heating section while being in direct or indirect contact with the conductive member and is heated by the heat from the conductive member. In FIG. 8, the conductive member and the material to be heated are not shown.
The first to third divided exciting coil units A, B, and C are arranged in series in a direction crossing (intersecting) the feeding direction of a material to be heated. Line Oxe2x80x94O represents a reference line of paper passing for the material to be heated. P1, P2, and P3 represents the paper passing widths relative to the reference line Oxe2x80x94O in which materials to be heated of three sizes, namely, large, medium-sized, and small materials, are passed. The paper passing widths P1, P2, and P3 have a relationship P1 greater than P2 greater than P3.
The sum of the lengths of the first to third divided exciting coil units A, B, and C substantially corresponds to the large paper passing width (maximum paper passing width) P1, the sum of the lengths of the first and second divided exciting coil units A and B substantially corresponds to the medium-sized paper passing width P2, and the length of the first divided exciting coil unit A substantially corresponds to the small paper feeding width P3.
The coils 20a, 20b, and 20c of the first to third divided exciting coil units A, B, and C are independently and selectively energized according to the width of a material to be passed therethrough.
That is, in a case in which a large material is passed, the coils 20a, 20b, and 20c of the first to third divided exciting coil units A, B, and C are energized corresponding to the large paper feeding width P1, and the conductive member generates heat in the large paper feeding width P1 so as to heat the large material.
In a case in which a medium-sized material is passed, the coils 20a and 20b of the first and second divided exciting coil units A and B are energized corresponding to the medium-sized paper passing width P2, and the conductive member generates heat in the mediums-sized paper passing width P2 so as to heat the medium-sized material.
In this case, the current to be applied to the coil 20c of the third divided exciting coil unit C corresponding to the paper nonpassing region is controlled (the supply of power is cut off or the amount of power to be supplied is reduced) so that the portion of the conductive member corresponding to the paper nonpassing region does not generate heat.
In a case in which a small material is passed, the coil 20a of the first divided exciting coil unit A is energized corresponding to the small paper passing width P3, and the conductive member generates heat in the small paper passing width P3 so as to heat the small material.
In this case, the currents to be applied to the coils 20b and 20c of the second and third divided exciting coil units B and C corresponding to the paper nonpassing region are controlled so that the conductive member does not generate heat in the paper nonpassing region.
The above-described control makes it possible to prevent a paper nonpassing region overheating phenomenon when medium-sized or small recording materials are passed.
The present invention has been made in view of the above problems, and an object of the present invention is to provide an improved electromagnetic induction heating apparatus in which efficiency is improved by reusing demagnetized energy to heat a material in order to prevent overheating in a paper nonpassing region, and in which overheating can be prevented with simple means.
Another object of the present invention is to provide an induction heating apparatus which provides a high power consumption efficiency.
In order to achieve the above objects, according to an aspect of the present invention, there is provided an induction heating apparatus including a heating member, a first coil for generating a magnetic field so as to induce an eddy current in the heating member, a second coil for cancelling the magnetic field generated by the first coil; and a third coil connected to the second coil and wound in a direction opposite from the winding direction of the second coil.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.